The instant invention relates to high temperature seals and more particularly to a high temperature low friction dynamic seal for use in applications, such as in the exhaust system of an automobile.
It is generally recognized that the fuel efficient engines of most modern automobiles operate at substantially higher temperatures than the less efficient engines of older automobiles. In this regard, it is well recognized that the operating temperature of an internal combustion engine is directly related to combustion efficiency and that as a result, an engine which is designed for maximum fuel efficiency inherently operates at higher temperatures than a less efficient engine. Further, the temperatures of the exhaust gases emitted by a fuel efficient engine are normally substantially higher than the temperatures of the exhaust gases from a less efficient engine. For this reason, the various components of the exhaust systems of most modern automobiles must generally be capable of withstanding prolonged exposure to substantially higher temperatures than the components of the exhaust systems of older automobiles.
In addition to the above, it has been found that in many cases it is advantageous to mount the engine of a front wheel drive automobile in transverse relation. However, it has been further found that when an engine is transversely mounted in an automobile it is generally necessary for the exhaust system of the automobile to be at least partially flexible in order to accommodate normal movement of the engine during acceleration and deceleration. Specifically, it has been found that it is generally necessary to provide at least one flexible joint in the exhaust system of a vehicle having a transversely mounted engine in order to compensate for relative movement between the engine and the vehicle chassis. Further, it has been found that in order to provide an effective leak-proof joint of this type for a vehicle having a relatively fuel efficient engine it is necessary for the joint to include a low friction seal which is capable of withstanding temperatures of at least approximately 1400.degree. F. However, while various seals have been heretofore available which are capable of withstanding temperatures in this range, virtually all of the heretofore available seals for applications of this type have been found to produce objectionable squeaking noises when they are used in flexible joints in exhaust systems.
The instant invention provides an effective dynamic seal which can be utilized in a flexible joint of an exhaust system of a modern automobile without producing objectionable squeaking noises. Specifically, the seal of the instant invention is capable of effectively sealing a flexible joint in a high temperature exhaust system of a modern automobile without producing squeaking noises. In particular, the seal of the instant invention comprises a compressed wire mesh base portion and a high temperature resistant low friction dry rubbing composite or shell portion on the base portion. The base portion is preferably formed in a circular ring-like configuration from compressed knitted wire mesh and the shell portion is preferably applied to the outer surface of the base portion. The shell portion comprises between approximately 2% and 30% by weight of vermiculite having an aspect ratio (length or width/thickness) of greater than approximately 10 and a mean particle size of less than approximately 50 microns. The shell portion further comprises between approximately 10% and 88% by weight of high temperature resistant fibers having a melting point of at least approximately 1400.degree. F. and between approximately 10% and 88% by weight of a solid lubricant consisting of one or more materials selected from a group consisting of barium fluoride, calcium fluoride, cerium fluoride and a eutectic of barium fluoride and calcium fluoride. The shell portion is preferably compressed on the base portion so that it forms a smooth low friction mating surface on the seal and the shell portion preferably further comprises between approximately 1% and 30% by weight of a mixture consisting of graphite and an oxidation inhibitor selected from a group consisting of aluminum metaphosphate and titanium diboride, wherein the weight ratio of graphite to oxidation inhibitor in the mixture is between approximately 50:1 and 1:1. The seal preferably further comprises a metal sheet having a melting point of at least approximately 1400.degree. F. on the inwardly facing side of the base portion and the vermiculite preferably has a mean particle size of less than approximately 10 microns. The fibers in the seal preferably consist of fibers, including whiskers, selected from a group consisting of metal fibers having a melting point above approximately 1650.degree. F., alumina fibers and boron fibers and in the preferred embodiment of the seal the fibers comprise stainless steel fibers.
In order to form a seal of the above type in accordance with the method of the instant invention a compressed wire mesh base portion is first formed and thereafter a formable composition is applied to the base portion to form a shell portion thereon. The formable composition which is applied to the base portion preferably comprises between approximately 5% and 65% by weight of a mixture consisting of between approximately 4% and 40% by weight of water and between approximately 60% and 96% by weight of vermiculite having an aspect ratio of at least approximately 10 and a mean particle size of less than approximately 50 microns. The formable composition further comprises between approximately 10% and 88% by weight of fibers selected from a group consisting of alumina fibers, boron fibers and fibers of one or more metals having a melting point of at least approximately 1400.degree. F., and between approximately 10% and 88% by weight of a solid lubricant selected from a group consisting of barium fluoride, calcium fluoride, cerium fluoride and a eutectic of barium fluoride and calcium fluoride. After the formable composition has been applied to the base portion, the base portion with the formable composition thereon is heated to evaporate substantially all of the water from the formable composition and the water free formable composition is then compressed to form a compressed shell portion on the base portion having a substantially smooth outer surface thereon. In particular, both the base portion and the shell portion are preferably compressed so that the total volume of the voids in the entire seal structure is less than approximately 50 % of the seal volume and so that the total volume of the voids in the shell portion is less than approximately 40% of shell portion volume. The formable mixture which is applied to the base portion preferably further comprises between approximately 1% and 30% by weight of a mixture consisting of graphite plus an oxidation inhibitor selected from a group consisting of aluminum metaphosphate and titanium diboride wherein the ratio of graphite to oxidation inhibitor is between approximately 50:1 and 1:1. Further, the step of forming the base portion preferably includes a step of applying a metal sheet to the inwardly facing side of the wire mesh ring before compressing the wire mesh ring to form the base portion. The metal sheet preferably has a melting point of at least approximately 1400.degree. F. and preferably at least approximately 1650.degree. F. Further, the vermiculite preferably has a mean particle size of less than approximately 10 microns and the fibers in the composite material preferably have a melting point of at least approximately 1650.degree. F.
Accordingly, it is a primary object of the instant invention to provide an effective high temperature seal for a flexible joint for an automotive exhaust system.
Another object of the instant invention is to provide a high temperature seal which can be effectively utilized in a flexible joint in an exhaust system without producing squeaking noises.
An even further object of the instant invention is to provide a method of making an improved high temperature seal for an exhaust system.
An even still further object of the instant invention is to provide an effective method of making a seal which can be utilized in a flexible joint in a high temperature exhaust system without producing squeaking noises.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.